The present invention relates to a method of generating an image scanning clock signal in an optical scanning device.
There have been known optical scanning devices in which a light beam is cyclically deflected as a scanning beam that scans a given information storage surface to read information therefrom or write information thereon.
Some of such optical scanning devices employ a rotating light deflector as a means for cyclically deflecting the light beam.
The rotating light deflector comprises a rotating polygonal mirror or a hologram disc composed of a holographic diffraction grating, which is rotated to deflecting the light beam. Where the light beam is deflected by the rotating light deflector, the repetitive light beam deflection does not occur in a uniformly periodic pattern because of manufacturing errors of the rotating polygonal mirror or the hologram disc or mechanical errors arising from mechanical rotation of the rotating light deflector.
Starting points of a scanning region, i.e., positions where respective scanning cycles are started, should be aligned with each other on the surface which is scanned by the scanning beam. If such starting positions were not aligned accurately, then an image written on the surface would be distorted by jitter, or an image reconstructed from read-out signals would be distorted by jitter.
One way of aligning the scanning starting positions is to position a light sensor outside of the scanning region, detect the scanning beam moving toward the scanning region each time the scanning beam is deflected, thereby generating a synchronizing signal, count clock pulses of an image scanning clock signal up to a prescribed number by using the synchronizing signal as a reference signal, and effecting a light scanning cycle after the clock pulses have been counted up to the prescribed number. Therefore, when the synchronizing signal is generated, the image scanning clock pulses are counted up to m clock pulses, and the scanning cycle is started at the time the (m+1)th clock pulse is reached.
Since the image scanning clock pulses are successively produced, the sychronizing signal would be generated at different times with respect to the image scanning clock signal if the synchronizing signal were produced irregularly due to an error of the rotating light deflector. It is assumed that an image scanning clock pulse is counted when the image scanning clock signal changes from the "low" state to the "high" state. If the synchronizing signal is generated immediately before the image scanning clock signal changes from the low state to the high state, then one clock pulse is counted when the image scanning clock signal goes high. If the synchronizing signal is generated immediately after the image scanning clock signal changes from the low state to the high state, then first one clock pulse is counted when the image scanning clock signal changes from the next low state to the high state. Therefore, the image scanning starting points can be varied to an interval which is equal to at most one image scanning clock pulse.
The image scanning clock signal is used as a reference for optical scanning of the information storage surface, and the width of one clock pulse is equal to the width of one pixel of the image to be read or written by the scanning light beam. With the above process of aligning the scanning starting points, therefore, the scanning starting points are subject to a variation up to one pixel width, and the image which is written or read out suffers a corresponding amount of jitter. Jitter-induced image distortion would be considerably noticeable if it were equal to the width of one-half pixel or more, with the result that the reproduced image would be much less appealing aesthetically to the eye.
Methods of reducing variations of the light scanning starting points are disclosed in Japanese Kokais 51-89346 and 56-126378. The method disclosed in the former publication requires a reference clock signal having a frequency n times higher than that of the image scanning clock signal in order to suppress variations of the light scanning starting points down to an interval of 1/n pixel or smaller. As the extent to which starting point variations can be reduced is proportional to the frequency of the reference clock signal used, this method is disadvantageous in that its ability to reduce the starting point variations is limited by the reference clock signal frequency that can be achieved.
The method disclosed in the latter publication is affected by the allowable operation error of a delay element used. In order to attain a desired degree of reduction of the starting point variations, such a delay element error has to be reduced to a certain range, an effort which results in a high cost.